Preparing 29.97 and 30 fps video for VSE
You will need to conform the footage to a single base rate. Although being very close, the values will be significantly off enough to cause sound slippage.

Regarding the Apple device, if from North America, I have extreme doubts that the footage is a base 30 rate. Typically most North American devices will list 24P or 30P and yet still be NTSC 24000/1001 and 30000/1001 respectively.

FFMPEG is probably a great entry point to conform any footage that requires such. Bear in mind that this may well have adverse effects on your footage as the frames in question may be dropped or doubled up depending on which target frame rate you are conforming to.